This invention relates to a monochrome cathode ray tube, and more particularly relates to an electron gun for such tube having improved high voltage performance.
Monochrome cathode ray tubes, for example, for projection television, employ a single electron gun mounted in the neck of the tube to focus a single electron beam on the fluorescent display screen of the tube. A deflection yoke surrounding the neck of the tube and associated electronic circuitry causes the beam to scan the screen as well as to vary in intensity in response to a video signal to produce a monochrome display image.
In projection color TV, three such displays, each in one of the primary colors red, blue and green, are superimposed on a large projection screen to produce a full color display image. Because the images on the tube screens are not viewed directly, but are magnified and projected by a system of projection lenses, the individual cathode ray tubes are driven at higher voltages and beam currents than would be encountered for direct view tubes, in order to produce a full color display of acceptable brightness. This higher brightness must be achieved without significant degradation of image resolution. Thus, it is essential that the tubes' electron guns exhibit not only good high voltage performance but also satisfactory lens performance.
Good high voltage performance is defined herein as a high threshold of field emissions during tube operation. Such a high threshold can be achieved if, for example, the electron gun is susceptible to effective high voltage conditioning. High voltage conditioning is carried out as one of the final steps of the manufacturing process, and constitutes subjecting the electron gun to voltages in excess of those encountered during subsequent tube operation, in order to induce arcing between the components to eliminate particles, projections and other sources of stray emission.
The most critical part of such an electron gun structure for both high voltage performance and lensing performance is the high voltage gap between the final focusing and accelerating electrodes. In an electron gun design of the prior art, this gap is defined by partially overlapping focusing and accelerating electrodes, the focusing electrode comprising a cup-shaped top portion and an adjoining elongated cylindrical base portion having a diameter of about 56% of that of the cup-shaped portion. The accelerating electrode comprises a taller and wider cup-shaped top portion coaxially surrounding the cup-shaped portion of the focusing electrode, and a shorter and wider cylindrical base portion, coaxially surrounding the upper region of the cylindrical base portion of the focusing electrode.
While it has been found in practice that the lensing performance of such an electron gun is acceptable for the demanding application of projection television, nevertheless the high voltage performance is less than desired for such an application.